The present invention relates to surface measurement technology, and more particularly to a surface measuring method and apparatus using a low energy neutral particle (neutral atom, neutral molecule) beam.
With recent development of devices having a solid body surface, ultra thin film (super lattice film) and the like, a desire to measure the chemical bonding state of surfaces is increasing more and more. Particularly, a desire to measure the chemical bonding state of small surface areas (characteristic length representative of the dimension of an area to be measured, is shorter than or equal to 1 .mu.m, or more severely 1 nm) is increasing nowadays.
As conventional methods for measuring the chemical bonding state, there are known a method using an electron beam such as Electron Energy Loss Spectroscopy (EELS), a method using an ion beam such as Ion Scattering Spectroscopy (ISS), and a method using an X-ray such as X-ray Photoemission Spectroscopy (XPS). These methods are described in "Structure and Dynamics of Surfaces I", edited by W. Schommers and P. von Blanckenhagen, Springer-Verlag, Tokyo (1986), respectively at pp. 245 to 276 for EELS, at pp. 56 to 61 for ISS, and at p. 63 for XPS.
With the EELS method, an electron beam of several tens eV is applied to a specimen surface, and the energy of reflected electrons is analyzed to determine the chemical bonding state of atoms and molecules on the surface. With the ISS method, an ion beam of several keV is applied to a specimen surface, and the energy of reflected ions is analyzed to measure the surface chemical state. With the XPS method, an X-ray of several hundred eV to several keV is applied to a specimen surface, and the energy of photoelectrons emitted from the surface is analyzed to determine the surface chemical state. These methods, although they provide the surface chemical state, are associated with the following problem. Namely, a specimen surface is damaged (chemical bonding state is changed) during the measurement because of excessive energy of a particle beam (electron beam, ion beam) or an X-ray incident to the surface. This problem essentially arises because the energy of such a particle beam is larger than the intensity of the general chemical bonding state which is 1 to 10 eV.
In order to realize non-destructive surface measurement, the energy of an incident particle beam should be maintained lower than or equal to 1 eV. It is practically difficult, however, to obtain such a low energy electron or ion beam having charged particles. The reason for this is a difficulty in controlling the energy and locus of a particle beam due to distributed space charges of a flux of flying particles having electric charges.
On the other hand, in the case of an electromagnetic wave (light wave in broad sense) such as an X-ray, it is easy to obtain a low energy beam smaller than or equal to 1 eV. However, this is also associated with the following problem. Specifically, the energy E and wavelength .lambda. of light are related to each other by: EQU .alpha.=ch/E (1)
wherein c represents the velocity of light, and h represents Planck's constant, so that the smaller the energy E becomes, the longer the wavelength .lambda. becomes. The relationship between the energy E and wavelength .lambda. of light is shown by a line labeled as "LIGHT" in FIG. 2. As seen from FIG. 2, a light wave having an energy E equal to or smaller than 1 eV has a wavelength equal to or longer than 1 .mu.m (i.e., in the infrared region). A light wave having a wavelength equal to or longer than 1 .mu.m cannot be converged smaller than about 1 .mu.m or less because of diffraction and interference. In other words, the method using a light wave (electromagnetic wave) cannot satisfy at the same time both the non-destructive nature of E.ltoreq.1 eV and the capability of measuring a small area for .lambda..ltoreq.1 .mu.m.
A particle beam (electron, ion, atom, molecule, and the like) other than a light wave can also be considered as a wave from the standpoint of quantum mechanics, so that the kinetic energy E and de Broglie wavelength of a particle beam are related to each other by: EQU .lambda.=h/.sqroot.2mE (2)
where m is the mass of a particle. As understood from the equation (2), even with the same energy E, the wavelength becomes longer as the mass m becomes smaller. The relationship represented by the equation (2) for an electron beam is shown by a solid line indicated "ELECTRON" in FIG. 2. In this case, an electron beam having an energy E equal to or larger than about 10.sup.-5 eV has a wavelength equal to or shorter than 1 .mu.m, thereby allowing non-destructive small area measurement. An electron beam having a wavelength equal to or shorter than 1 nm, however, requires the energy E which is equal to or larger than 1 eV, resulting in a possibility of surface damage of an ultra-small area equal to or shorter than 1 nm.
As described above, surface measurement using a light wave or charged particle beam may cause surface damage, and it is very difficult to non-destructively measure the chemical bonding state of a small area.